The present invention generally relates to an electrical filter device which employsdielectric coaxial resonators as filter elements and more particularly, to a filter of the above type having incorporated therein a lowpass filter having a spurious wave eliminating function.
Generally, as an electrical filter device, there has been known an arrangement which includes a plurality of dielectric coaxial resonators as filter elements and a substrate for coupling, incorporated into a casing, a plurality of capacitor electrodes formed on said coupling substrate and individually connected to inner conductors of the respective dielectric coaxial resonators, and coaxial connectors attached to the outer wall of said casing as input and output ports.
In the known filter as described above, there is a problem with respect to its performance, namely that, for example, when the resonators incorporated as the filter elements are 1/4 wavelength type, the spurious wave suppression ratio for the higher harmonic which becomes an odd multiple wave of a fundamental wave is deteriorated, while when such resonators are of 1/2 wavelength type, the spurious wave suppression ratio for the higher harmonic which becomes an integral multiple wave of the fundamental wave is also deteriorated.
The electrical filter device of the above described type does not normally have a spurious wave eliminating function in itself, and therefore, in such a filter device, it has been a conventional practice to eliminate the spurious waves by externally connecting thereto, circuit components having the spurious wave eliminating function. In the conventional practice as described above, however, there is the disadvantage that additionanl circuit components are required, besides the original parts constituting the filter device, thus resulting in an undesirable increase in the number of parts involved.
In order to overcome the disadvantage as described above, filter devices have been conventionally proposed in which low-pass filters having a spurious wave eliminating function, or band cut-off filters, are incorporated as spurious wave eliminating filters, as shown in FIGS. 17(A), 17(B) and 17(C). In FIG. 17(A), a spurious wave eliminating filter (SEF) 4.sub.0 is connected to each of two band-pass filters (BPF) 5.sub.0 which are coupled in parallel relation to each other. In FIG. 17(B), a spurious wave eliminating filter (SEF) 4.sub.0 is connected to a junction of two band-pass filters (BPF) 5.sub.0 connected in parallel to each other, while in FIG. 17(C), a spurious wave eliminating filter (SEF) 4.sub.0 is connected in series with a band-pass filter (BPF) 5.sub.0.
In connection with the above, a low-pass filter is normally constituted by connecting inductors L1.sub.0, L3.sub.0 and L5.sub.0 to a plurality of capacitors C2.sub.0 and C4.sub.0 employing a single dielectric member 1.sub.0 as shown in FIG. 16(a), or by connecting inductors L1.sub.0 ', L3.sub.0 ' and L5.sub.0 ' to a plurality of capacitors C2.sub.0 ' and C4.sub.0 ' employing a plurality of dielectric members 2.sub.0 and 3.sub.0 as illustrated in FIG. 16(B). In the constructions as described above, however, there is the problem that electromagnetic waves tend to leak therefrom, and unless a shielding function is imparted by some other means, high frequency characteristics are liable to be deteriorated.
On the other hand, a band cut-off filter is mainly so arranged as to produce a trap function at a particular frequency through utilization of a coaxial cable of a predetermined length. However, the band cut-off filter as described above can not remove spurious waves at a band region outside the predetermined cut-off region.
Besides the above, there is available a system which is adapted to impart a ground stray capacity to the resonators, thereby to deviate the band region for spurious waves, but by this system, the spurious waves themselves are not attenuated to a large extent, and therefore, effective elimination of spurious waves can not be expected.
As described so far, the conventional filter devices having the spurious wave eliminating function involve the problems that either the devices tend to be large in size on the whole, or sufficient spurious wave eliminating function can not be achieved.